Japanese Patent Application Publication No. 2017-180320 describes an evaporated fuel processing device. The evaporated fuel processing device comprises a canister, a vent passage, a pump, and a detector. The canister retains evaporated fuel generated in a fuel tank. The vent passage communicates the canister and an intake passage of an engine. The pump is arranged on the vent passage. The pump discharges mixed gas including air and the evaporated fuel retained in the canister to the intake passage. The detector detects a difference between a pressure of the mixed gas discharged by the pump in the vent passage on an intake passage side relative to the pump and a pressure thereof in the vent passage on a canister side relative to the pump (hereinbelow the difference will be termed a pressure difference). A concentration of the evaporated fuel in the mixed gas is estimated from the detected pressure difference.